Countershaft transmissions, double-clutch gearboxes, automatic transmissions, manual transmissions or similar devices known from the prior art are usually designed with gears rotatably mounted on a shaft that engage with other gears non-rotatably mounted on a shaft, thereby forming gear pairings. By alternating non-rotating connections between gears mounted rotatably on the shaft, it is possible to connect the gear pairings to form different gear ratios of a countershaft transmission in the drive train of a countershaft transmission by means of appropriate shifting elements, such as synchronizations, dog-clutches or frictional elements.
The shifting elements provided for connecting the gear pairings are mechanically, hydraulically, pneumatically or magnetically actuated internally or externally in relation to the shaft on which the rotatably mounted gears or idlers of a countershaft transmission are arranged.
DE 102 06 584 A1, DE 43 25 964 A1 as well as DE 103 02 502 A1 represent the general state of the art as it applies to actuating shifting elements from the interior of a shaft outward for the purpose of engaging and disengaging idlers or gear pairings.
Actuating shifting elements of a transmission device externally is customarily performed using shift forks or similar devices, while the actuation of the shifting elements from within the shaft holding the idlers characteristically requires less radial construction space compared to actuating the idlers externally in relation to the surface of the shaft, owing to the shifting action within the shaft. In contrast thereto, transmission devices in which the idlers are actuated from within require more axial space than transmission devices in which the shifting elements are actuated externally.
This results from the fact that the actuating force required for actuating the idlers from within the shaft holding the idlers must be conducted to the shifting elements also arranged on the shaft holding the idlers and interposed between the idlers. Since neither the bearing width of the idlers arranged on the shaft nor the axial space required by the shifting elements can be modified and the guidance of actuating force through the shaft into these areas is only possible under certain conditions, the shifting elements must be actuated from within in areas of the shaft that are not functionally stressed by the idlers or shifting elements. This means that free areas of the shaft are required which, however, can only be made available by elongating the shaft.
Making the gear pairings of a preselector gearbox toothed at an angle to minimize noise during transmission device operation further increases the amount of axial space required for a transmission device. The increased axial space requirement results from the fact that an axially shifted axial force is conducted into the gears in addition to the normal and peripheral forces in the area of the activated gear pairings toothed at an angle, by means of which torque is transmitted from one shaft to another shaft of a transmission device.
When a transmission device is designed with asymmetrically arranged shafts, tilting moments resulting from the axially displaced, axial forces exerted on the activated gears, thereby causing a skewing of the gears. This type of angled gearing leads to undesired noise increases during the operation of the transmission device as well as shortened service life, whereby both of these phenomena increase as skewing becomes more pronounced, for which reason gear skewing must be limited to only a few angular minutes.
Gear skewing is minimal, if the gears are mounted on the shafts by means of sufficiently stiff bearings, whereby this requirement can be implemented in the design of fixed gears more easily than in the case of idlers. To create idler bearings with the desired stiffness, the idler bearings must be designed with appropriate bearing widths. For reasons of cost, idler bearings are generally realized as wide needle bearings, sometimes as double-rowed needle bearings, whereby the required bearing widths of the idlers have a negative influence on the overall length of the transmission and a transmission device requires a very large amount of axial space when shifting elements are actuated from within, i.e. from the shaft outward.